The present invention relates to a modified polypropylene. More particularly, the present invention relates to a modified polypropylene of a particular composition distribution and molecular weight distribution, a process for producing such modified polypropylene and an article produced from such modified polypropylene.
Polypropylene refers to that class of polymers derived primarily from propylene. The term "polypropylene" is generally understood to include both the homopolymer of propylene and "modified" polypropylene such as copolymers of propylene and blends of polypropylene with minor amounts of other components. The present invention is primarily concerned with such "modified" polypropylenes.
Polypropylene has found a wide variety of commercial uses in today's society such as, for example, candy wrappers, potato chip bags, diapers, carpet backing, battery cases and household containers to name a few.
Polypropylene is produced by polymerizing propylene and, optionally, minor amounts of other comonomers such as, for example, other alpha-olefins like ethylene, 1-butene and 1-hexane, at various temperatures and pressures in the presence of one of many well-known transition metal halide catalysts generally referred to as Ziegler or Ziegler-Natta catalysts. By proper selection of the catalyst and operating conditions, a highly stereospecific polymer can be produced.
For the case of homopolypropylene, every second carbon atom is asymmetric being bound not only to two chain methylene carbon atoms but also to one hydrogen and one methyl group. The hydrogen and methyl groups of the asymmetric carbon can lie in various arrangements in planes above and below the plane defined by the backbone carbon atoms.
"Isotactic" refers to the situation when all of such methyl groups lie in the same plane, this plane being opposite to the plane occupied by the hydrogen groups. The concentration of "meso diads" ([m]), which is the fraction of adjacent pairs of repeat units having the same configuration, is a quantitative measure of the degree of isotacticity of the polypropylene.
"Syndiotactic" refers to the situation where such methyl groups (and, consequently, such hydrogen groups) of consecutive asymmetric carbon atoms lie in alternating planes. The concentration of "racemic diads" ([r]), which is the fraction of adjacent pairs of repeat units having opposite configurations, is also used to describe the degree of stereoregularity in polypropylene.
"Atactic" refers to the situation when the hydrogen and methyl groups lie randomly in the planes above and below the asymmetric carbon. For atactic polypropylene, [m] is generally about equal to [r].
Highly isotactic homopolypropylene is characterized by a high value of [m], a high degree of crystallinity and a low portion of material that is soluble in hydrocarbon solvents at or slightly above room temperature. Desirable properties of these highly isotactic homopolypropylenes include high tensile strength, hardness and high melting point. Undesirable properties include brittleness and low impact strength, particularly at lower temperatures. Highly isotactic homopolypropylene is also difficult to process in, for example, film applications requiring biaxial stretching of the polymer.
It is currently possible with the latest generation of propylene catalysts to produce crystalline homopolypropylenes of greater than 99% meso diads ([m]&gt;0.99). In earlier stages of catalyst development, meso diads in the low 90% range were common. Because of their high isotacticity and crystallinity, these new highly isotactic homopolypropylenes have even greater modulus, tensile strength, hardness and melting point properties than their prior art counterparts, but they suffer even more from relatively poor brittleness, low impact strength and processing problems.
It would, therefore, be highly desirable to produce a polypropylene which incorporates the best of the modulus, tensile, hardness and melting point properties characteristic of high isostatic content while concurrently improving upon the brittleness, impact and processing properties of the polymer. The present invention provides such an improved polypropylene by modifying a highly isotactic polypropylene to "plan" its composition, molecular weight distribution and crystallinity. Thus, the present invention is directed to modified polypropylenes having an unexpected brittleness, impact and processing properties associated with polypropylene copolymers while unexpectedly retaining the tensile and hardness properties associated with crystalline homopolypropylene.
Polypropylene has, in the past, been modified in a variety of ways in an attempt to alter its properties to fit a desired end use. For example, it is well known to copolymerize propylene with minor amounts of other comonomers, as well as to blend homopolypropylene with other polymers and copolymers such as, for example, propylene-ethylene copolymers, to improve impact strength and brittleness. To a certain extent, these copolymers and blends do improve the undesirable properties of highly isotactic homopolypropylene, but often at the expense of the desirable properties.
For example, prior art copolymers of propylene and other alpha-olefins generally do not combine the best properties of each homopolymer, with the presence of the other olefin negatively affecting modulus and tensile properties of homopolypropylene. Other polypropylene modifications suffer from the same shortcomings. A number of these well-known polypropylene modifications are taught by the following references, all of which incorporated by reference for all purposes as if fully set forth: U.S. Pat. Nos. 3301921, 3358056, 3401212, 3414637, 3529037, 3737557, 3817783, 3954704, 4245062, 4301256, 4339557, 4365045, 4414369, 4499247, 4492787, 4500682, 4510292 and 4526931.